It doesn't happen that often, but we've all been in a situation where we need to measure something and don't have a ruler. Luckily, you probably already have a "good enough" ruler in your pocket: a dollar bill.

One US dollar bill is six inches long, so you can just use that to get a ballpark estimate of whatever it is you need to measure. You can also transfer this logic to pretty much anything else you'd have on hand: your phone (an iPhone 4 is 4.5 inches long), a piece of paper (8.5 by 11), or the length of your phone's charging cable. It may sound silly now, but measure the stuff you carry with you—one day, you'll have that eureka moment where it comes in handy.